1. Field of the Invention
The present invention generally relates to circuits which measure the amount of unbalance in a balanced circuit, such as transmission lines, and more particularly relates to longitudinal balance measuring bridge circuits.
2. Description of the Prior Art
For reasons of convenience and cost, balanced lines such as twisted pair, are much preferred in data transmission to single-ended wiring, such as coaxial cables. Depending on the degree of balance between the pairs, minimal electromagnetic radiation of the transmitted signal and pick-up of interfering signals are achieved without complex shielding. A balanced transmission system, shown in FIG. 1, consists of two conductors, A and B, neither of which is grounded. We define two voltages:
The transmitted signal, i.e. the voltage, VT, between A and B is known as “transverse” or, in old telephone parlance, “metallic.”
The algebraic average of VA and VB, i.e.VL=(VA+VB)/2is known as “longitudinal” or “common mode”. This is the voltage one would measure from the electrical midpoint between A and B to ground. Such a midpoint is most easily established by a center-tapped transformer connected between the balanced terminals, as shown in FIG. 1.
Presence of common mode voltage is not necessarily harmful. However, a given signal having both transverse and longitudinal voltage components indicates unbalance, i.e. either signal has been partially converted into the other. Picked-up common mode voltage converted into a transverse voltage will cause interference and transverse voltage converted to longitudinal voltage will result in radiation.
ITU-T, the Telecommunication Standardization Sector of the International Telecommunications Union, has issued Recommendation O.9, entitled “Measuring Arrangements to Assess the Degree of Unbalance about Earth.” In it, the ratio of an applied longitudinal signal to the resulting unwanted transverse signal is defined as “Longitudinal Conversion Loss (LCL).” The recommended test bridge configuration to measure LCL is shown in FIG. 2.
Although, for the sake of generality, the item under test (IUT) 2 is a two-port network, the measurement can be performed only on one port at a time.
The longitudinal signal, G, is applied to the balanced circuit via a center-tapped auto-transformer. The magnitude of G is measured by voltmeter VL1, while voltmeter VT1 measures the resulting transverse voltage. Z1 is the characteristic impedance of the balanced circuit. ZL1, the common mode source impedance, is specified as Z1/4. Z2 is the termination impedance of the IUT, especially when twisted pair, coaxial cable or other transmission line is tested. The test circuit of FIG. 2 clearly shows the elements required for the balance measurement, which are:
1. Common mode signal source;
2. Common mode signal injection circuit;
3. Balanced line termination; and
4. Transverse voltage monitor.
Aside from showing what has to be measured where, the circuit does not prescribe how to perform the measurement in practice.
A conventional circuit to measure the LCL of a 150 ohm balanced IUT 2 in conjunction with a 50 ohm network analyzer 4 is shown in FIG. 3. It consists of common mode signal injection transformer T1, common mode choke T2 and balun T3. The common mode input signal is terminated in 50 ohm. Balun T3 transforms the 150 ohm balanced IUT impedance to the 50 ohm coaxial input of the network analyzer 4. Since the network analyzer input sees VT2 rather than VT1, its reading has to be corrected to display VL1/VT1.
Among the limitations of the conventional circuit are the following:
It is essential for the bridge to respond only to the transverse signal and not to the common mode signal that may leak through. Therefore, common mode attenuation is crucial. However, much of the load of discriminating against common mode is borne by balun T3, where it is rather difficult to assure superior common mode rejection over the frequency range of interest.
Since there exists a need for longitudinal balance bridges to measure IUTs of different balanced impedances, the conventional circuit will require a multiplicity of balun designs to accommodate this range of impedances.
Calibration to assure accuracy of measurement is rather difficult in the conventional bridge. The correction factor will differ from unit to unit and vary over the frequency range particularly at the high and low ends.
The ITU-T recommendation 0.9 also specifies a two-port measurement of longitudinal balance in paragraph 4.3. This measurement is called “longitudinal conversion transfer loss (LCTL)”. As mentioned previously, the one-port measurement is known as “longitudinal conversion loss (LCL)”.
The measurement configuration for LCTL is shown in FIG. 4.
A common mode signal, VL1, is applied to port ab of an item under test (IUT) 2 via a center-tapped auto-transformer. As mentioned previously, in the one-port method of measurement (FIG. 2), the transverse signal, VT1, due to unbalance, is measured at the same port ab and the ratio, VL1/VT1, is defined as the “longitudinal conversion loss (LCL)”.
In the two-port method (FIG. 4), the transverse signal, VT2, is measured at port de, and the ratio VL1/VT2 is defined as the “longitudinal conversion transfer loss (LCTL)”.